Monoethylene glycol (MEG) and monopropylene glycol (MPG) are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focussed on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols can be seen as an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to sugars revolve around a hydrogenation/retro-aldol process as described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513.
An important aim in this area is the provision of a process that is high yielding in desirable products, such as ethylene glycol and propylene glycol, and that can be carried out in a commercially viable manner. A preferred methodology for a commercial scale process would be to use continuous flow technology, wherein feed is continuously provided to a reactor and product is continuously removed therefrom. By maintaining the flow of feed and the removal of product at the same levels, the reactor content remains at a more or less constant volume.
Continuous flow processes for the production of glycols from saccharide feedstock have been described in US20110313212, CN102675045, CN102643165, WO2013015955 and CN103731258. A process for the co-production of bio-fuels and glycols is described in WO2012174087.
Typical processes for the conversion of saccharides to glycols require two catalytic species in order to catalyse retro-aldol and hydrogenation reactions.
Typically, the hydrogenation catalyst compositions tend to be heterogeneous. However, the retro-aldol catalysts are generally homogeneous in the reaction mixture. Such catalysts are inherently limited due to solubility constraints. Further, the saccharide-containing feedstock is generally in the form of a slurry in a solvent or as a homogeneous saccharide solution. The handling of such a reaction mixture requires careful consideration. Slurry reactors and ebullated bed reactors are taught as preferred options for a one-pot saccharides to glycols process in US20110313212 and WO2013015955, in order to deal with such considerations.
It is known that thermal degradation of reaction intermediates can occur in the conversion of saccharides to glycols. Such degradation reduces the overall yield of desired products and increases the complexity of the isolation process of said desired products. It has generally been found that carrying out the reaction with high concentrations of starting materials in a reactor exacerbates this degradation and the formation of by-products.
Typically, the conversion of saccharides to glycols has, therefore, been carried out as a continuous flow process with a high degree of back mixing using a saccharide-containing feedstock comprising a low concentration of saccharide in solvent. The process is carried out in the presence of usually more than one catalytic species capable of catalysing retro-aldol and hydrogenation reactions. Such a process may be carried out, for example, in a continuous flow stirred tank type reactor. In such a system, the concentration of reactants at any one point will be low, preventing any decomposition due to high concentrations. However, in such a process, as some of the back-mixed reaction mixture is continuously removed from the reactor, there will be some material that does not react to completion. This results in a product stream that contains starting material and/or intermediates, reducing the overall yield of the process and requiring separation of the starting material/intermediate from the desired product and disposal or recycling thereof.
One solution to overcome this issue was disclosed in WO2015028398, which describes a continuous process for the conversion of saccharides to glycols, in which the retro-aldol and hydrogenation reactions take place first in a stirred reactor, from which a product stream is continuously removed. The product stream is then provided to a finishing reactor, which operates essentially in a plug flow manner. As a high degree of conversion is achieved in the first reactor, the product stream entering the plug flow reactor will inevitably have a low concentration of starting materials present therein and thermal degradation is, thus reduced.
Another method is described in CN102731258, which describes a reactor in which there is suspended a catalyst filter basket in a position higher than the level of liquid reagents. The reagents are injected into the catalyst basket where they are contacted with hydrogenation catalyst compositions and then travel through the stirred slurry reactor in the bottom of the reactor vessel before flowing out of the bottom of the reactor. Said reactor vessel is equipped with a recycle loop from which reagents are re-injected into the catalyst basket.
US 2015/0329449 describes a process in which carbohydrates which can yield aldoses are reacted in a reactor having a first zone comprising mostly a retro-aldol catalyst and a second zone that contains a reducing catalyst. In said process, the aldoses are at least partially converted into glycolaldehyde in the first zone. The glycolaldehyde is then converted to ethylene glycol in the second zone of the reactor.
Further optimisation of a process for the conversion of saccharides into glycols is always desirable. It would be preferable to carry out a continuous process to provide glycols from saccharide-containing feedstock in as high a yield as possible. In such a process, it is desirable that substantially full conversion of the starting material and/or intermediates is achieved and formation of by-products is reduced. Minimising the complexity of any reactor system would also be beneficial.